1. Field of the Invention
The present invention relates to liquid crystal displays. More specifically, it relates to reducing flicker in reflective liquid crystal display devices.
2. Discussion of the Related Art
Producing a color image using a Liquid Crystal Display (LCD) is well known. Such displays are particularly useful for producing images that are updated by frames, such as in color televisions. Typically, each image frame is composed of color sub-frames, usually red, green and blue sub-frames.
Such LCD systems employ a light crystal light panel that is comprised of a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprised of pixel rows and pixel columns. To produce a desired image, the individual pixel elements are modulated in accordance with image information. Typically, the image information is applied to the individual pixel elements by rows, with each pixel row being addressed in each frame period.
Pixel element matrix arrays are preferably xe2x80x9cactivexe2x80x9d in that each pixel element is connected to an active switching element of a matrix of such switching elements. One particularly useful active matrix liquid crystal display is the reflective active-matrix liquid crystal display (RLCD). An RLCD display is typically produced on a silicon substrate and is often based on the twisted nematic (TN) effect. Thin film transistors (TFTs) are usually used as the active switching elements. Such RLCD displays can support a high pixel density because the TFTs and their interconnections can be integrated onto the silicon substrate.
FIG. 1 schematically illustrates a single pixel element 10 of a typical RLCD. The pixel element 10 is comprised of a twisted nematic liquid crystal layer 12 that is disposed between a transparent electrode 14 and a pixel electrode 16. For convenience, FIG. 1 shows the transparent electrode applied to a common ground. However, in practice the transparent electrode is usually biased, say at +7 volts. Additionally, a storage element 18 is connected to complementary data terminals 20 and 22. The storage element receives control signals on a control terminal 24. In responsive to a xe2x80x9cwritexe2x80x9d control signal the storage element 18 selectively latches the voltage on one of the data terminals 20 and 22, and applies that latched voltage to the pixel electrode 16 via a signal line 26. The voltages on the data terminals 20 and 22 are complementary. That is, if the transparent electrode is at ground, when one line is at +2 volts, the other is at xe2x88x922 volts. Still referring to FIG. 1, and as explained in more detail subsequently, the liquid crystal layer 12 rotates the polarization of the light 30, with the amount of polarization rotation dependent on the voltage across the liquid crystal layer 12. Ideally, the pixel element 10 is symmetrical in that the polarization rotation depends only on the magnitude of the latched signal on the signal line 26. By alternating complementary signals in consecutive frames, unwanted charges across the liquid crystal layer 12 are prevented. If only one polarity was used, ions would build up across the capacitance formed by the transparent electrode 14, the liquid crystal layer 12, and the pixel electrode 16. Such charges would bias the pixel element 10. Thus, the pixel elements are driven by complementary signals in consecutive frame periods. Those frame periods are thus grouped into even frames and odd frames, with the even and odd frames being interlaced.
The light 30 is derived from incident non-polarized light 32 from an external light source (which is not shown). The non-polarized light is polarized by a first polarizer 34 to form the light 30. The light 30 passes through the transparent electrode 14, through the liquid crystal layer 12, reflects off the pixel electrode 16, passes back through the liquid crystal layer 12, passes out of the transparent electrode 14, and then is directed onto a second polarizer 36. During the double pass through the liquid crystal layer 12 the polarization of the light beam is rotated in accord with the magnitude of the voltage on the signal line 26. Only the portion of the light 30 that is parallel with the polarization direction of the second polarizer 36 passes through that polarizer. Since the passed portion depends on the amount of polarization rotation, which in turn depends on the voltage on the signal line 26, the voltage on the signal line controls the intensity of the light that leaves the pixel element.
The storage element 18 is typically a capacitor connected to a thin film transistor switch. When a control signal is applied to the gate electrode of the thin film transistor that transistor turns on. Then, the voltage applied to the source of the thin film transistor passes through the thin film transistor and charges the capacitor. When the control signal is removed, the thin film transistor opens and the capacitor potential is stored on the pixel electrode 16.
FIG. 2 schematically illustrates a pixel element matrix. As shown, a plurality of pixel elements 10, each having an associated switching thin film transistor and a storage capacitor, are arranged in a matrix of rows (horizontal) and columns (vertical). For simplicity, only a small portion of a matrix array is shown. In practice there are numerous rows, say 1290, and numerous columns, say 1024. Referring to FIG. 2, the pixel elements of a row are selected together by applying a gate (switch) control signal on a gate line, specifically the gate lines 40a, 40b, and 40c. A constant voltage (which is shared by all of the pixel elements) is applied to the transparent electrode 14 from a ramp source 41 via a line 42. Furthermore, the ramp source 41 applies complementary ramp signals on lines 20 and 22 (which are also shared by all of the pixel elements 10). Furthermore, column select lines 46a, 46b, and 46c, control the operation of the pixel elements 10.
A row of pixel elements is selected by the application of a signal on an appropriate one of the gate lines 40a-40c. This turns on all of the pixel elements in that row. Then, the ramp source 41 applies a ramp to either line 20 or line 22 (which line is used is varied in each frame). The ramp begins charging all of the storage capacitors in the selected row. As the other rows are not energized, the ramp source only charges the OFF-state capacitance of the other pixels. When the ramp voltage reaches the desired state for a particular pixel, the column select line (46a-46c) voltage for that particular pixel element 10 turns the pixel switch OFF. Then, the ramp voltage that existed when the particular pixel element 10 was turned OFF is stored on that element""s storage capacitor. Meanwhile, the ramp voltage continues to increase until all of the column select lines (46a-46c) cause a ramp voltage to be HELD on an associated pixel element. After that, a new row of pixel elements is selected and the process starts over. After all rows have been selected, the process starts over again in a new frame period, this time using the complement of the previous ramp.
The foregoing process is generally well known and is typically performed using digital shift registers, microcontrollers, and voltages sources that are beneficially fabricated on a common substrate using semiconductor processing technology on polysilicon and/or amorphous silicon.
While RLCD displays are generally successful, they have their problems. For example, in practice the pixel elements are not ideal in that the polarization rotation of the light 30 is not symmetrical. That is a +1 volt signal does not necessarily produce the same rotation as a xe2x88x921 volt signal. While the physical principles behind this asymmetry are not fully understood, it appears that one explanation for this phenomenon is that the liquid crystal layer 12, the transparent electrode 14, and the pixel electrode 16 interact to form a battery that biases the pixel electrode 16 relative to the transparent electrode 14. Compounding the problem is the fact that the bias is not constant over time or temperature, and that the bias changes due to manufacturing variations. The result is ion movement that produces a DC voltage offset. The DC voltage offset produces gray scale distortions and limits the achievable gray scale range.
Additionally, the bias offset introduces intensity xe2x80x9cflickerxe2x80x9d that causes problems that are difficult to correct for. While visual flicker can be minimized by increasing the frame rate such that the human eye does not perceive the flicker, that flicker negatively impacts peak contrast, intensity, and color. Therefore, a technique of correcting for DC bias and/or flicker induced problems would be beneficial.
Accordingly, the principles of the present invention are directed to a method and to an apparatus of reducing flicker. According to the principles of the present invention a light beam is modulated by image information organized by frames. In alternating frames, designed as even and odd frames, that light beam is modulated use complementary voltages taken relative to a common (transparent) electrode. The difference in the average modulation in the even frames and in the odd frames over time is then determined. That difference is then used to adjust the potential of the common electrode such that the difference in the average modulation over time is zero.
The apparatus includes a beam splitter for passing light having a first polarization and for deflecting light having a second polarization. Polarized light from the beam splitter is modulated by a liquid crystal display driver comprised of a plurality of pixel elements connected in a pixel matrix. Each pixel element includes part of a common transparent electrode, a pixel electrode, and an interposed liquid crystal layer. The liquid crystal display driver modulates its received light, by even and odd frames, to produce a modulated light beam. That modulated light beam is then directed through an optical system that optically modifies the modulated light beam. The optically modified light beam is then displayed on a viewing screen. A light sensor receives a portion of the modulated light beam and produces a sensor signal. That sensor signal is used by correction circuitry to sense a difference in the average modulation of the light beam in even and odd frames. The correction circuitry then adjusts the common transparent electrode potential such that the modulation in the even numbered frames and the modulation in the odd numbered frames have the same average modulation.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.